This invention relates generally to method and apparatus for measuring the flow velocity of a stream of fluid; and more particularly to method and apparatus for measuring the flow velocity of the stream of fluid flowing within the borehole of a well, traversing subsurface formations of the earth, by use of a nonradioactive tracer substance.
Determination of well fluid flow velocity, and the corresponding well fluid flow rate, has long been regarded as standard practice in well management as such information can be used to ascertain the contribution of various strata to the total well flow during production (or, conversely, the relative extent of fluid injection into various strata during secondary recovery); or to evaluate the efficiency of fluid circulation within a well, among other uses.
Several methods of determining borehole fluid flow velocity are commonly used, and the invention described herein possesses distinct advantages over many of the methods. As prominent as any among these methods is that of injecting as slug of radioactive tracer substance into the stream of fluid flowing within the borehole and calculating the length of time required for the tracer material to flow a known longitudinal distance within the well to a radioactivity detector, or between two lonitudinally spaced detectors. This method is objectionable, and thus not favored by well operators, because of the concern of contaminating fresh water strata with radioactive matter, and the difficulty and expense of locating and segregating the radioactive matter at the well head and thereafter properly disposing of such matter. One of the several features, and advantages, of the present invention, is to therefore furnish a method of measuring borehole fluid velocity involving timed travel in the fluid stream flowing within the borehole of a nonradioactive tracer substance.
Another velocity measuring method which involves timing the travel of a detectable substance in the fluid stream flowing within the borehole is activation of oxygen atoms by neutron bombardment, which yields a radioactive isotope of nitrogen. The limitations of this method, to use with fluids containing water, is not possessed by the invention described herein; and one of the several features, and advantages, thereof is to furnish a method of measuring the borehole flow velocity of an aqueous or nonaqueous fluid involving the timed travel in the field stream flowing within the borehole of a nonradioactive substance.
Borehole fluid velocity can also be determined with a spinner flowmeter, wherein a blade mounted externally on a logging tool is caused to rotate by the fluid flow within the borehole, and measurement of the rate of rotation of the blade allows calculation of the borehole fluid velocity. However, the use of a spinner flowmeter is limited to those circumstances in which the fluid flowing within the borehole is traveling at a rate sufficient to rotate reliably the spinner blade. One of the several features, and advantages, of the invention described herein is that it can be used to measure borehole fluid flow velocity when the stream flowing within the borehole does not have sufficient velocity to rotate reliably a spinner blade.
Accordingly, the present invention furnishes improved methods and apparatus for measuring the velocity of the stream of fluid flowing within the borehole of well by detecting the travel time therein of a nonradioactive tracer substance.